The present invention generally relates to heat treatment and more particularly, to a method of gas carburizing and hardening steel articles and a continuous furnace therefor.
It has been conventionally known that processes of gas carburizing and hardening of steel articles comprise a heating step of heating the steel articles to a carburizing temperature, a carburizing step of holding the steel articles in a carburizing atmosphere for a predetermined period of time so as to cause carbon to be absorbed into surfaces of the steel articles, a diffusing step of diffusing the absorbed carbon into the steel articles, and a hardening step of cooling the steel articles so as to harden the steel articles. Meanwhile, the processes of gas carburizing and hardening the steel articles are classified, in accordance with pressures in furnaces at the carburizing step and the diffusing step, into a gas carburizing and hardening method in which the steel articles are heat treated in the vicinity of atmospheric pressure by using an endothermic atmosphere or a mixture of N.sub.2 gas and a hydrocarbon gas, and a vacuum carburizing and hardening method in which the steel articles are heat treated at subatmospheric pressure by using a mixture of N.sub.2 gas and a hydrocarbon gas or the hydrocarbon gas only. The gas carburizing and hardening method has such an advantage as to enable a continuous furnace to have a simple construction but is disadvantageous not only in that the processed steel articles assume so-called carburizing colors such as a grayish brown color, a grayish black color, etc. but in that a quenching media becomes rapidly deteriorated, thereby resulting in a short life thereof. On the other hand, in the vacuum carburizing and hardening method, the steel articles have bright surfaces without assuming the carburizing colors and the quenching media has a long life. However, the vacuum carburizing and hardening method has such an inconvenience that since a plurality of chambers each separated by a vacuum partition door from one another are required to be provided in order to produce a continuous furnace, the continuous furnace becomes complicated in structure.
Meanwhile, the above described carburizing colors are produced by chromic oxides formed on the surfaces of the processed articles during the gas carburizing process or soot adhering to the surfaces of the processed articles during the gas carburizing process. It is known that when an article having a carburizing color is heated at about 900.degree. C. in a vacuum of 10.sup.-1 to 10.sup.-2 torr, dissociation of oxygen is effected due to drop in partial pressure of oxygen such that the processed article has a bright surface.